The helical antenna and its derivatives have been widely used in the field of communications for several decades. The helical antenna typically operates in the frequency ranges of 1 GHz and above. Many helical antennas resemble a coil of wire in the shape of a spring. As shown in FIG. 1A, wrapping the antenna wire on a cylinder is a relatively easy way to form the coils of a helical antenna. As shown in FIG. 1B, the cylinder used to construct the element is often incorporated into and becomes part of the antenna system. Helical antennas that operate at frequencies below 1 GHz tend to be more difficult to manage and construct because of their physical size and weight. For example, a 2 GHz helical antenna has a coil diameter of 2 inches while a 500 MHz helical antenna has a coil diameter of 8 inches. As a general rule, the larger coils of conventional helical antennas are supported by means of a main rod located in the center of the coil with smaller rods supporting the element.
The hemispherical helical antenna is a derivative of the helical antenna. Typically, hemispherical helical antennas exhibit a gain of approximately 8 dbi and are much smaller in axial length than a cylindrical helical antenna of similar gain. However, the coil shape of the hemispherical helical is significantly more challenging to form and support than the cylindrical helical antenna.